Radio telephone system



May 10 1932- F. F. MERRIAM 1,857,819-

` RADIO TELEPHONE SYSTEM Filed Junezo, 1928 2 'sheets-sheet 1 A TTU/WVEY May 10, 1932. l F. F. MERRIAM 1,857,819

' RADIO TELEPHONE sYsTEM Filed June 20, 1928 2 Sheets-Sheet 2 5M 0/ FMejHR/AM by?? ATTRNEV Patented May 1 0, 1932 FRANCIS F. MERRIAM, OF BLOOMFIELD, NEW JERSEY, ASSIGNOR T BELL TELEPHONE LABORATORES, NGOBJPORATED, OF NEW YORK, N. Y., A ACOE??(lSRiJlllON OF NEW YORK Application filed June 20,

This invention relates to radio telephone systems and has for an object to simplify the calling or signaling circuits of such systems.

Une method which has been found satisfactory for signaling in'connection with radio telephone transmitters'consists in generating an audio frequency calling tone and employing that tone for modulating the radio frequency carrier wave.

Heretofore, this method of signaling has required the use of a separate calling tone generator, or additional elements adapted to be associated with those included in the circuit-s of an amplifier or modulator and switching means for connecting the additional elements into the tube circuits, whereby the amplifier or modulator operates to generate low frequency oscillations which are used to modulate the carrier for calling purposes.

In the specific embodiment herein shown and described this invention comprises a radio telephone transmitter employing modulator circuits of the so-called Heising constant current type, disclosed in U. S. Patent 1,442,147, January 16, 1923. The radio frequency carrier current is generated by a space discharge oscillator, controlled by a piezo electric crystal, and amplified in an electric v discharge amplifier.

Sil

Voice frequency waves are impressed upon the input circuit of an electric discharge modulator, the plate of which is connected t0 the plate of the radio frequency amplifier lirough a high frequency choke coil.

T he amplifier and modulator are supplied with space current from a common source through a low frequency choke coil which operates to maintain constant the total direct current supplied to the amplifier and modulat-or.

Villien voice frequency waves are supplied to the input ofthe modulator, voice frequency currents are established in the output circuit and, due to the effect of the constant current choke coil, the current supplied to the space path of the amplifier is varied in accordance with the voice frequency currents, thus producing voice frequency variations in the amplitude of the radio frequency waves supplied to the antenna.

RADIO TELEPHONE SYSTEM 1928. Serial No. 286,852.

For call signaling purposes a relay is provided to'connect a condenser in shunt to a portion of the low frequency or constant current choke coil and to connect the tuned'circuit thus formed across the input electrodes of the modulator. ln this manner the modulator is caused to produce oscillations in the audio frequency range which are usedl to modulate the radio lfrequency carrier in the same way as the voice frequency currents. Thus, a portion of the constant current choke coil is associated with a condenser and is transferred to the input circuit of the modulating tube which'operates to generate a call ing signal tone.

At the receiving station the telephone and calling signal receiving circuits are connected together and to the output of the radio receiver. The call signal receiving circuit vis tuned to be sharply selective of the calling tone, while the switchliook for the telephone receiver is provided with a contact for shortcircuiting the telephone receiver when the switchhook is down. rlhis provides a very simple method of eliminating interference between the call signal and telephone receiving circuits.

The invention will be more readily understood by reading the following detailed description in connection with the drawings in which Figs. 1 and 2 placed end to end show a combined transmitting and receiving radio telephone system embodying the features of this invention. i

Fig. 1 shows a radio telephone transmitter and Fig. 2 shows a radio telephone receiver both of which are adapted to be controllably connected to an antenna 1.

Power for energizing the tubes of the two circuits is supplied from a double current generator having a 400 volt armature 2 and a 24: volt armature 8.

The transmitter circuit comprises a piezo electric crystal controlled space discharge oscillator a, a radio frequency amplifier 5 and a modulator 6.

The oscillator i is a shielded unit comprising a three-electrode vacuum tube 7 having a piezo-electric crystal 8 connected between its cathode and grid for controlling the fre* quency of the waves generated. The piezoelectric crystal 8 is maintained at a substantially constant temperature by means of an electric heating coil 26 connected to the 24 volt armature 3 through a thermostat 27.

The primary Winding 9 of an output transformer 10 is connected between the anode and cathode of the tube 7. The secondary winding 11 of the transformer 10 is connected to the three-electrode vacuum tube 12 employed in the amplifier circuit 5. One terminal of the secondary winding 11 is connected through a blocking condenser 13 to the grid of the tube 12. The other terminal of the winding 11 is connected through a neutralizing condenser 14 to the plate of the tube 12, and the mid-point of the winding 11 is connected t-o the cathode of tube 12. This type of circuit is disclosed and claimed in Rice Patent 1,334,118 of March 16, 1920. When the constants of the circuit are properly proportioned, the production of parasitic oscillations in the circuit of tube 12 is prevented.

The primary winding 9 of the transformer l consists of two coils placed one on each side of the secondary winding 11, the-two halves being wound in opposite directions and connected in parallel aiding relation. A symmetrical winding arrangement of this kind results in a balancing of the capacity between the windings so that both ends of the secondary coil have the same capacity to ground. By the use of this type of transformer the adjustment of the neutralising condenser 14 is made substantially independent of frequency over a comparatively wide range of frequencies.

The output circuit of the amplifier comprises a tuned plate circuit for the tube 12 consisting of variable condensers 16 and 1"? and inductance 18. Condenser 1'? also serves as a coupling condenser to the antenna 1. A loading coil 19 and an antenna. tuning condenser 20 are included in the antenna circuit.

The modulator circuit 6 is of the constant current type disclosed and claimed in the above noted Helsing patent. This circuit comprises a three-electrode vacuum tube the grid and cathode of which are connected to the secondary winding of an input transformer 23.

The plate of the tube 22 is connected to the plate of the amplifier tube 12 through a hi gli frequency choke coil 24.

Space current for the tubes 7, 12 and 22 is supplied through a low frequency constantcurrent choke coil 25 as hereinafter described in detail.

Heating' current for the cathodcs of tubes 7, 12 and 22 is supplied from the 24 volt armature 3.

The tube 7 is continuously energized to generate oscillations. 'Under normal operating conditions the power supply to the tubes 12 and 22 is disconnected, and heating current is supplied to the cathode of tube T orer the following circuit From the positive terminal of the armature 3, through switch resistance 29, resistance 30, left-hand contact and armature of relay 31, and cathode of tube l' to the grounded negative terminal of the armature 3.

lV hen the relay 31 is actuated by removing the receiver from the switchhoolr of the telephone set, as hereinafter described, the armature of relay 31 is moved to engage with the right-hand contact, so that the connection through resistance to the cathode of the tub-e 7 is broken. lll/Then this occurs, heating current is supplied to the cathode or" tube 7 as follows: from the positive terminal of the armature 3, through switch 23, resistance 29, cathode of tube 22, cathode of tube 12, righthand contact and armature of relay 31, and

athode of tube 7 to the grounded negativo terminal of armature 3.

lzlate current for the tubes 12 and 22 is supplied from the 400 volt armature 2 through an inductance-capacity filter which serves to suppress connnutator ripples. The negative terminal of the armature is connected to the series cathode circuit through conductor 32 and resistances 33 and Space current is supplied to the tube 7 from t ie positive terminal of the armature 2 through filter 35, conductor 3G, right-hand armature of relay 3. choke coil conductor 33 and primary winding 9 to the plate of the tube, across the space path within the tube to its cathode and via yground resistuices 34 and 33, conductor 32 to the negative terminal of the armature 2. llVhen no signals are being transmitted the plate current supply for the tubes 12 and 22 is disconnected.

The system is provided with a push button. As will be hereinafter described, when the push button is actuated the relay 39 is ene-rgized to connect the transmitter to the antenna 1 by means of its upper contact, and its lower contact completes the circuit from the upper terminal of the retard coil to the common plate circuit of the tubes 12 and Resistance 43 and condenser 4e prevent arcing at the contacts of relay 39.

lesistances 33 and connected in the common space current circuits of the tubes l'. 12 and 22 between the negative terminal of armature 2 (through lead 32) and the series cathode circuit at ground 34a.

The grids of tubes 7 and 22 are connected to the terminal of resistance 34. nearest the negative terminal of armature 2 thro tard coil 40 and resis ice in series for 2' and the resistance 50 for tube The grids of these tubes are thus maintained at a negative potential with respect to the cathodes by the potential crop in resistance Similarly, the grid of the tube 12 is connected to the terminal of resistance 33 nearest the negative terminal of armature 2 through retard coil 42, maintaining that grid at a negative' potential by the drop in both resistances 33 and 34. Condenser 51 provides a yby-pass around resistances 33 and 34 for alternating currents.

The actual'biasing potential applied to the grid of the tube 12 is increased by the voltage drop across the filament of tube 7 and the biasing potential applied to the grid of tube 22 is augmented by the voltage drop across the filaments of tubes 7 and 12.

To produce a calling signal tone the relay 37 is energized over a circuit to be hereinafter described. The operation of this relay transfers the positive high voltage lead 36 from the lower terminal of the coil 25 to the mid-terminal 45 thereof, connect-s the series circuit including condenser 46 and resistance 47 in shunt to the lower half of the coil 25, and connects the grid of the tube 22 through resistance 48' and stopping condenser 49 to the lower terminal of coil 25.l Under these conditions, condenser 46 andthe lower half of coil 25 form a resonant grid circuit for thev tube 22, tuned to the required frequency for the calling signal, and a feed backv connection is provided due to the coupling kbetween the two halves of the coil 25.`

The tube 22, therefore, operates as a tuned grid circuit oscillator producing the desire-d calling tone, which modulates thehigh frequency output of amplifier tube 12 in the. same way as does the voice output of the tube 22 when it is operating as an amplifier.

The receiving circuit shown in Fig. 2 is of the double detection type comprising a heterodyne oscillator 60, a highfrequency detector 61, an intermediate frequency amplifier 62, an audio-frequency detector 63, an audiofrequency amplifier 64 and a signal frequency amplifier 65.

The receiving circuit is connected to the antenna 1 through a conductor 66 and the upper contact and upper armature of relay 39. A 'variometer 67 is provided for timing the antenna circuit. Two parallel resonant circuits 68 and 69 coupled through a condenser 70 provide high frequency selectivity. The selective circuits are coupled to the antenna through the condenser 71.

The heterodyne oscillator circuit 60 is of the type described and claimed in the copending application of H. T'. Friis, Serial No. linicie, filed Api-ii ce, 192e.

The oscillator circuit comprises the threeelectrode vacuum tube 72, the plate and grid of which are connected together through a tuning condenser 73. An inductive winding 74 is connected in shunt to the condenser. Inductive windings 75 and 76 are connected in the cathode heating current leads and are closely coupled to the winding 74 to provide an effective connection between Vthe windingv 74 and the cathode. These windings also serve to maintain the alternating current potential of the cathode above ground, thus permitting the rotary plates of the tuning condenser 73 to be maintained at ground p0- tential,A whereby the tuning of the oscillator may be adjusted without introducing disturbancesidue to the capacity-to-ground of diate-frequency amplifier 62 through a transformer 80.

The intermediate frequency amplifier mayv comprise one or more amplifier stages. The

output of the last stage is coupled to the grid circuitof a vacuum tube detector 81 employed in the audio-frequency detector circuit 63 through an intermediate-frequency filter 82. The output of the detector is coupled to the audio-frequency amplifier 64 through a transformer 83.

The output circuit of the audio-frequency amplifier 64 comprises a telephone receiver 84 and the primary winding of an output transformer 85 connected in series. The secondary winding of the transformer 85 is connected to the input of the signal frequency amplifier 65, the output of which is connected through a transformer 86 to the signal receiving circuit 87, to be hereinafter described.

Cathode heating current, space current and grid biasing potential for the tubes inciuded in the receiving circuit are supplied from the high voltage armature 2, through the leads 88 and 89. The cathode heating circuit may be traced from the lead 88, through resistance 90, resistance 91 and cathode of tube 81v in parallel, resistance 92, resistance 93, resistance 94 resistance 95, resistance 96, resistance 97, resistance 98, resistance 99, lead 103, resistance 100, retard coil 101 and switch 102 to the positive lead 89. v The cathodes of the tubes in the signal-frequency amplifier, audio-frequency amplifier, intermediate-frequency amplifier, high frequency detector and heterodyne oscillator are connected in shunt to the resistances 94, 95, 96, 97 and 98, respectively.

The plates of the oscillator tube, the high frequency detector tube, and the various anipliier tubesv are connected to the lead 103,

through the resistances 104, 105, 106, 107

and 108, respectively. The plate of the audiofrequency detector tube is connected through conductor 109, middle contact of jack 110 and lresistance 111 to the common terminals of' the resistances97 and 98.

rlhe grid of the oscillator tube 72 is connectedV to the negative side of the cathode of that tube to give a desired biasing potential. The biasing potential for the grid of the tube iso 8lr-isdeter-mined byfthezdropf in resistance 90,1.

tliatllior thel audio-frequency Yamplifier `tube bythedropin the resistance9a,`and thatfforf-t` the-signal4 frequency ainpliiier lbythe drop iu' 'resistance 93.

GrainA regulation is obtained by*l means et a circuit ofIthe `type disclosed and claimed in the copen'ding applicationV ot-Bruce, Serial No.'158,169, tiledDecem'berBL 1926. ln thisl Y, circuit, regulation Ais obtained by deteriniin4 jack l110` to the terminal of the resistance 111',-

adjacent `the plate of the detector tube 81.

-B means of this connectiony an increase in the strength of the received signal Wave Will tend to increase the plate current of the detector 63, 'thus raising the negative bias on tliefgridor'the detectortube 79 and driving the operating point offt'ha't tube toward cut oli1 on its' grid 'voltage-platecurrent charac teristic.l

ByV properly selecting the value'ot resistance 111, thewaudioetrequency current, supplied to the signaling devices '84' and 113, may be'maintainedsubstantially constant in spite of theta-ct that fading may cause the received high frequency Wave to vary `in amplitude over a relatively Wide range. f

The automatic. gain regulatoris rendered ino'perativewby inserting` a plug in the jack 110,5andy rthe'valuey of the biasing potential applied to tliegrid ofthe detector tube-79 islixed. Vhen thefplug is inserted, 'the cir- .cuit fthroughresistance 1111 `isl opened, 'and the grid ofthe detectortube 79 is connected tothe power' supplycircuit'atthe common terminals of the i'esistafnces 94 and 95 through the .loiverfcontactfof .jack-'1103 This procedure is'desirablevvhen tlie .heter-odyne oscil-k later 60 is to be adjusted.

The tone signallreceiving` circuit 87 is of the-type` described and claimed in Demarest et al1, `Patent 1,657,498`f'otdanuaryl, 1928,

to Which reference isinade for Ia inorede#` tailed description. l/Vfhen the combination ot pulses to which the selector 112 is adjusted is received, theselector is stepped around to closerra circuitfrom ground through its conrtactarm,'winding of bell 113. resistance 114,

contact 115 of telephone switchhoolc 116, conductor-117s to the positive terminal of lon7 voltage-armature 3. The bell`113 istliereby energized to indicate that the station isbeing called.

The telephone set comprisesa transmitter 118', va receiver 84 and a switchhoolr 116. The alternating current circuit of the transmitter comprises condenserV 119 and the primary WindingI of transformer 120. The secondary Winding of transformer 1120i vis "connected, u

throughuthe' leads 12:1 tol .theprimaryWind-2/ ing of the input transforiner'23 of the ampli-"- er 6 included in the transmitting circuit! When the receiver 84 is removed from the switchhoolr, direct current is suppliedtothe transmitter 118 StromV the-lead 117, through u, switchhook 116', resistance 122 and retard coil f 123m series. f Sin'iultaneousl i o eratin Vcuis;-

.la P g rent issupplied from lead 117 through the switchliook and klead A124: to relay 31, vto en:

ergizethe transmittingcircuit; -VVhen vit is desired to talk, .push'zbutton-125 .is operated to supply operating current to relayr99 fromv lead 117, through connection. 12.6.

lhen the receiver is-placed onlthehook the above mentioned circuits-'are opened and contact 127 is closed to short circuit the re*- cei-ver,so that it does not'respond to received calling signals;

To initiate a call the operator removes the receiver from the svvitchliooleand listens to make sure that the radio channel is not busy.

The operation 4of .thesvvitchhook energizes the transmitting circuit as above described. After determining that the channel is tree,

the operator actuates the signaling-'keyl28/- which is provided Withftvvocontacts'.

llVhen the key 128 'is actuated one oit its `contacts Vcloses and remains closed to' completeH `a circuit, While tlieother contact'operates f .to intermittently close andk open a second cire- Icuit in theA desiredrsequence tot supply `code impulses corresponding to the station to.' be

called.

The'rst contact.I e., between the arm'129 'and the drum of the key, is yin'parallel with the push button 125 and serves to complete the circuit through"the-antenna yrelay 39, l'whereby the tra-nsmitteris connected tothe antenna 1.

rlhesecond contact, i. e., between-the arm 129 'and' the contact'lSO,l serves to completen the circuit of .the relay. 37 :and therebyvchanges the connections ottliewampliiier 6.3 The amplifier is therefore caused vto oscil-f:

late and generate thev calling signal tone as above described. When the :key returns toitsv zero position lboth contacts are opened and the antenna relay is released.

Vihiat is claimed is:

1. 1n a Wave transmitting' system an electric discharge device for supplying high fre'-v 'quency Waves, a variableimpedance device having` an input .circuit and an output circuit ahr arranged to supply low frequency Waves for modulating the high frequency Waves, a source for supplyingcurrent to the output circuit ot saidY variable'impedance device, an

inductance coily connected in series with said 'l source in said output-circuit, and means for connecting a portion ott said coil yin the input circuit o't said variable impedancedevi'ce to produce a low frequency signaling tone. 1

2. 1n a Waveftransniitting `systemean .elec.

tric discharge device for supplying high frequency Waves, a second electric discharge device having an input circuit and an output circuit and arranged to supply low frequency Waves for modulating the high frequency Waves, a source of space current for said devices, an inductance coil connected in series With said source in the space current path common to said devices, and means for connecting a portion of said coil in the'input circuit of said second device to produce a low frequency signalingtone.

3. In a Wave transmitting system, an electric discharge device for supplying a carrier Wave, a second electric discharge device having input and voutput circuits and arranged to supply a message wave for modulating the carrier Wave, a low frequency inductance coil, a source of current for supplying space current through said inductance coil to the space current path of said second device, capacity means, and switching means for associating said capacity means With a portion of said inductance coil and for connecting the combined capacity and inductance in the input circuit of said second device to cause the generation of a low frequency signaling tone for modulating the carrier Wave.

4. A radio telephone system comprising an electric discharge device for supplying a radio frequency carrier Wave, a second electric discharge device having an input circuit and an output circuit and arranged to supply speech Waves for modulating the carrier Wave, a low frequency inductance coil, a source of current for supplying space current through a circuit including said inductance coil to the space current path of said second device, a condenser, a relay for associating said condenser With a portion of said inductance coil and for connecting the combined condenser and inductance to the input circuit of said second device to cause the generation of a low frequency signaling tone, and a signaling key for causing the operation and release of said relay in a predetermined sequence.

5. A radio telephone system comprising an antenna, a transmitting circuit, a receiving circuit, and a relay for controlling the connection of said circuits to said antenna, said transmitting circuit comprising an electric discharge device for supplying a radio frequency carrier Wave, a second electric discharge device having input and output circuits arranged to supply speech Waves for modulating said carrier Wave, a loW frequency inductance coil, a source of current for supplying space current through a common circuit including said inductance coil to the space current paths of said devices, a condenser, a second relay for associating said condenser With a portion of said inductance coil and for connecting the combined condenser and inductance in the input circuit of said second device to cause the generation of a low frequency signaling tone, and a signaling key arranged upon the initiation of its operation to operate the first mentioned relay to connect the transmitting circuit to the antenna, and during its operation to alternately cause the release and operation of the second mentioned relay.

6. In combination, a telephone set comprising a telephone transmitter, a telephone receiver and a switchhook, an antenna, an electric discharge device for supplying radio frequency Waves to said antenna, a second electric. discharge device having an input circuit and an output circuit, connections from said transmitter to the input circuit of said second device, a source of current, an inductance coil through which space current is supplied from saidV source to said discharge devices, means for causing the loW frequency output of said second discharge device to modulate the high frequency output of said firstdischarge device, switching means for connecting a portion of said inductance coil in the input circuit of said second device whereby low frequency oscillations are produced for signaling, a radio receiving circuit, a calling signal circuit selectively responsive to the calling signals to the substantial exclusion of Waves of other frequencies, means connecting said calling signal circuit and said telephone receiver together to the output of said radio receiving circuit, and means operated by said sWitchhook for short-circuiting said telephone receiver when the switchhook is down.

In Witness whereof, I hereunto subscribe my name this 18th day of June, 1928.

FRANCIS F. MERRIAM. 

